With recent DDOS attacks increasingly using NTP as an attack vector, and one of Cloudflare’s clients recently having been hit with a DDOS attack just short of 400gbps, we believe it is necessary to summarise what’s been going on, how such attacks are made possible at all, and what the community, and providers can do to prevent or mitigate such attacks as best possible.
A concise overview by means of a CERT alert can be found here: https://www.us-cert.gov/ncas/alerts/TA14-013A.
Essentially, an attacker send a certain command to a vulnerable NTP server, using a spoofed source address. The command itself is very short and produces very little traffic. The response, however, is a lot larger, besides the response is going to be sent back to the spoofed source address. This response is typically about 206 times larger than the initial request – hence the name amplification – a very effective means to quickly fill up even very powerful internet pipes.
Cloudflare published a very interesting article as well, giving a quick overview about the most recent attack and the technology behind it: http://blog.cloudflare.com/technical-details-behind-a-400gbps-ntp-amplification-ddos-attack.
The recommended course of action here is to secure your NTP server (cf. https://isc.sans.edu/diary/NTP+reflection+attack/17300) , as well as ensure that spoofed packets do not leave your network. Sample procedures are explained at BCP38.info.